drum brake abruptly grabbing

What do you think might cause my front drum brake to change from a progressive feel to no progression and just locking up the front tire?

This happened to me last fall when I first got the bike running. At the time I'd just installed new stock Andover shoes and turned the drum. The brakes barley worked. But a couple times, as I pulled on the lever expecting to pull it all the way back to the grip, the brakes grabbed so hard with very little effort, and no progressive feel that it made the bike dive and the wheel lock up. This seemed to sort itself out after a few breaking sessions, and feel progressive again (although still crappy stopping power)

-Over the winter I've had the shoe material replace with better material, had them arched to the drum and installed the Andover performance kit. At first, spinning the front wheel while the bike was on the stand produced a progressive feel on the lever. Now for some reason, it feels progressive at first, then the lever seems to hit a soft spot and the brakes lock up.
Any insight would be appreciated.

hi roadrash,this is what used to happen with the green am4 linings,epecially first thing in morning when damp,went away after a few brake applications to warm things up,but these have not been available for a few years due to asbestos content so doubt that is the problem,have you got a good chamfer on the leading edges of the shoes? chris

chris plant said:

hi roadrash,this is what used to happen with the green am4 linings,epecially first thing in morning when damp,went away after a few brake applications to warm things up,but these have not been available for a few years due to asbestos content so doubt that is the problem,have you got a good chamfer on the leading edges of the shoes? chris

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Thanks Chris, that's a good question and yes. After posting this question I had a conversation with the tech that replaced the shoe material. he did mention putting chamfers on the leading edges as standard procedure. He had also never herd of this problem except maybe when someone reverses the direction of the rotation by flipping the wheel. your experience with the am4 linings might be the best explanation. If nobody has had this experience I might just need to work them in once the weather permits.
Cheers

Cable new? Known high quality? Lever pivots all properly greased? Did you anoint the brake cam shafts with luscious moly grease at assembly? The grabby clutch/brake linings when damp still rear their heads on my daily driver car. :|

I've had Norton and Triumph drums dump me grabbing brake and working it fine till it self energized while tire crossed sandy patch, ZIP. Might try feathering the leading shoe edges and checking witness marks to guide ya but does have a thermal expansion hint to ponder.

I'm absolutely gob-smacked that that turkey Italian 2LS front brake could lock up a wheel. In all my test riding miles, I was lucky to get more than a slight reduction in speed. It would initially start a skow-down, but increased load on the brake lever just resulted in it coming all the way back to the throttle twist-grip with no additional deceleration. It was downright dangerous and we test riders had a hard time convincing management that we had a problem.

As I recall, it wasn't until Peter Inchley had to brake hard on a MIRA test run that he recognised we had an issue. I remember doing brittle lacquer deformation tests on the brake and showing that the backplate was deforming under load and preventing adequate force being applied to the shoes.

I was in the US by the time they figured out a fix.

frankdamp said:

I'm absolutely gob-smacked that that turkey Italian 2LS front brake could lock up a wheel. In all my test riding miles, I was lucky to get more than a slight reduction in speed. It would initially start a skow-down, but increased load on the brake lever just resulted in it coming all the way back to the throttle twist-grip with no additional deceleration. It was downright dangerous and we test riders had a hard time convincing management that we had a problem.

As I recall, it wasn't until Peter Inchley had to brake hard on a MIRA test run that he recognised we had an issue. I remember doing brittle lacquer deformation tests on the brake and showing that the backplate was deforming under load and preventing adequate force being applied to the shoes.

I was in the US by the time they figured out a fix.

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By "figured out a fix" are you referring to the stiffener plate AKA performance kit, or disk brakes?
You live in Anecortes? I should ride up there this spring, buy you a pint and have you look at my build

I remember doing brittle lacquer deformation tests on the brake and showing that the backplate was deforming under load and preventing adequate force being applied to the shoes.

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I'm gob smacked at the clever low tech distortion/deflection detection. I'll have to try that on some frame areas, cool. Best wishes solving drums. The one I had on P!! worked pretty good to control but didn't have a brake switch in line.

I would check that the lining has not detached from the shoe. Been there done that on autos.